The present disclosure relates to an object inspection method and system. More particularly, the disclosure relates to a solder joint quality inspection system particularly useful for inspecting solder bumps or balls which is non-contact, non-destructive, and which can be used either during process development or during on-line manufacturing.
Computer chips are connected to computer boards, such as printed circuit boards (PCB""s), normally by soldering. For instance, conventional computer chips have been connected to such boards in the past by soldering a plurality of pins extending from the chip to the board. Presently, consumer demands are driving the current trend in the electronics industry to make products that are compact, high in density, light, and thin. These demands have created new chip interconnection methods. One such interconnection method is known as solder bump or ball technology. Flip chip, ball grid arrays, chip scales, and multi-chip modules each use small solder bumps underneath the chips for interconnection, making them superior in performance to other more conventional interconnection technologies.
Solder joint quality inspection for traditional interconnection technologies typically comprise visual inspection techniques. Unfortunately, visual inspection techniques are ineffective for inspecting solder bumps because these bumps are hidden from view when used to connect a chip to a board. Moreover, these techniques are unsuitable for on-line inspection in that they require a large amount of time and are susceptible to human error. Therefore, new techniques for detecting flip chip and BGA solder joint defects are needed.
Currently, there are three main techniques used to inspect the solder joint quality of solder bump connected chips. These include x-ray detection methods, acoustic microscopy, and functional testing methods. There are two main types of x-ray detection methods: laminography and microfocus radiography. The difference between these two hinges mainly on cost, complexity, and shadowing. Although x-ray laminography can produce images of cross-sections of a solder joint, it is cost prohibitive and time consuming. The x-ray radiography approach provides a means of looking through the chips and substrates to see the relative location and size of the solder bumps. However, because this method relies on changes in the thickness of the material through which the x-rays pass, poor connections, delaminations, and cracks are very difficult to detect. In addition, the images which are produced by this technique must be correctly interpreted. Extracting solder joint quality information from these images, is difficult, time consuming, and subjective, making the process difficult to automate.
Acoustic microscopes utilize high-frequency ultrasound to examine the internal features in materials and components. Defects such as preexisting voids or non-wet conditions can be observed. However, the ultrasonic imaging systems currently available in the market are destructive techniques because the board assembly must be immersed in water during the inspection process. As can be appreciated, this technique is also too slow for on-line inspection, normally requiring several minutes to image the solder bumps under each single chip. Because many solder joints are located near the edge of the chip, edge effects can distort the ultrasound, providing a poor image in the region of interest.
The most widely used on-line inspection techniques are functional testing methods, such as the flying probe or the bed of nails methods. In these techniques, a test fixture checks for electrical continuity and proper operation of the assembled board by comparing the electrical response at specific nodes of the board to previously determined values. However, unsoldered joints may still pass this test if mechanical contact exists, even though the joint may fail after a short service life because of cracks or partial connections.
From the foregoing, it can be appreciated that it would be desirable to have an on-line, high resolution, fast, low cost, non-contact, and non-destructive method and system for inspecting solder joint quality.
The present disclosure relates to an object inspection system. The object inspection system comprises an ultrasound source capable of exciting the object to be tested with a stimulus such that the object vibrates at an ultrasound frequency, at least one optical fiber optically connected to the ultrasound source and adapted to be positioned with its exit end in close proximity to a surface of the object to be tested, the at least one optical fiber delivering the stimulus to the object, a vibration sensing device adapted to sense the ultrasonic vibration displacements created in the object with the ultrasound source, and a system controller which receives the ultrasonic vibration data from the vibration sensing device.
In a preferred arrangement, the object inspection system comprises a solder joint inspection system for testing the integrity of solder joints used to connect a computer chip to a printed circuit board. When arranged as a solder joint inspection system, the system can comprise a laser capable of producing a pulsed laser beam used to excite the object to be tested such that the object vibrates at an ultrasound frequency, at least one optical fiber optically connected to the laser and adapted to be positioned with its exit end in close proximity to a surface of the object to be tested, the at least one optical fiber delivering at least a fraction of the pulsed laser beam to the object, a laser interferometer adapted to sense the ultrasonic vibration displacements created in the object with the ultrasound source, and logic configured to process the ultrasonic vibration data used to evaluate the object.
The present disclosure also relates to a method for inspecting an object that comprises exciting the object with a pulsed laser beam such that it vibrates at an ultrasonic frequency, the pulsed laser beam being delivered to a surface of the object with at least one optical fiber, sensing the vibrations of the object to obtain vibration displacement data at a plurality of discrete, predetermined points of the object and processing the vibration displacement data and comparing it to vibration displacement data of a non-defective object to determine whether the tested object is defective.
The features and advantages of the invention will become apparent upon reading the following specification, when taken in conjunction with the accompanying drawings.